1. Field of the Invention
The present invention relates to a focused ion beam apparatus for processing and/or observing a sample by focusing an ion beam on the sample.
2. Background Art
Focused ion beam (FIB) apparatuses are currently commercially used for microfabrication of a sample or observation thereof based on the irradiation of the sample with a narrowly focused ion beam. An ion beam in the presence of a magnetic field on its optical axis is deflected by the Lorentz force. Since the accelerating voltage of an FIB apparatus is normally on the order of several 10 kV, the beam spot could be in some cases displaced by several dozen μm or more even by the earth magnetism. Ga, which is conventionally used as an ion species, has two isotopes 69Ga and 71Ga, which have different degrees of deflection by the magnetic field. As a result, the Ga ion beam can in some cases separate into two beams. The beam also separates when the ion forms a cluster. If the beam spot has a displacement on the order of several dozen μm, the distance between the two, separated isotope ion beams could be on the order of 1 μm, depending on the difference in their mass/charge ratios. Such phenomenon must be avoided or suppressed in FIB apparatuses, which are used for microfabrication on the order of nanometers.
One of the simplest methods of eliminating a magnetic field on the optical axis is to magnetically shield the ion beam column with a magnetic body, and such method has conventionally been used. Patent Document 1 discloses a technique to provide even the tip portion of an FIB apparatus with magnetic shield.
However, it is difficult to provide a complete magnetic shield close to the sample. While a virtually complete magnetic shield could be provided by covering the entire apparatus, including the sample, with a magnetic shield, it is practically impossible to produce a magnetic shield with a completely shielded structure. In practice, the magnetic shield needs to be provided with openings, through which entry of magnetic field cannot be avoided. A typical FIB apparatus comprises an electrostatic deflector for irradiating a desired position of the sample with an ion beam. While this can be used to correct the displacement in the ion beam spot on the sample due to a magnetic field, it cannot prevent isotopes separation simultaneously.
Patent Document 3 discloses a means to actively deflect an ion beam using a Wien filter. This means, however, is used rather for actively separating the isotopes so as to cause unwanted isotope components to collide against a wall to eliminate them. Thus, the ion beam emitting opening is made very narrow, through which it is difficult to pass the ion beam in the presence of an external magnetic field. Thus, there is a need for the development of a technology to prevent the displacement of the ion beam spot and the separation of isotopes even when there is a magnetic field on the ion beam optical axis.
This is more important in the case of an FIB-SEM which combines an FIB column and an SEM column. The FIB-SEM is a recently commercialized apparatus that combines an observation SEM (Scanning Electron Microscope) and an FIB apparatus so as to allow the observation of a sample processed by the FIB apparatus at higher resolutions. Objective lenses in an SEM normally comprise electromagnets. In order to obtain higher resolutions, it is necessary to use a lens of the so-called semi-in lens type or the snorkel type, which produce a leakage of magnetic field toward the sample. Such magnetic field enters and reaches the FIB optical axis and therefore strongly deflects the ion beam, whereby, when the ion beam comprises beams having a plurality of mass/charge ratios, the ion beams are separated. Since the ion beam needs to be irradiated onto the sample located near the SEM objective lens, the optical axis of the ion beam cannot be sufficiently magnetically shielded. Further, placing the magnetic shield near the SEM objective lens disturbs the magnetic field of the SEM objective lens, thereby adversely affecting the resolution of the SEM.
When using the FIB-SEM that uses an SEM of the type that produces a leakage of magnetic field near the sample, a method is employed whereby, during microfabrication of the sample by the FIB apparatus, the magnetic field for the SEM objective lens is terminated, and the FIB is terminated while the sample is observed by the SEM. However, even when the exciting current to the SEM objective lens is terminated, the magnetic field remains and varies with time, producing a shift in the ion beam spot with time. Patent Document 2 discloses a technique to prevent this, which places a degaussing coil for eliminating the remaining magnetic field near the SEM objective lens. However, this technique requires degaussing the SEM objective lens whenever the SEM is switched to the FIB apparatus and is thus bothersome.
When the sample is microfabricated by the FIB apparatus while at the same time the sample is observed with the SEM, the out lens type, which does not produce a leakage magnetic field, has conventionally been used as the SEM objective lens. However, there is a growing need for higher SEM resolutions, and the use of the semi-in lens type of objective lens is becoming unavoidable. Thus, there is a need for a technology to realize an FIB apparatus and an FIB-SEM in which the ion beam does not develop the separation of isotopes, nor does the position of the ion beam spot on the sample change, even in the presence of a magnetic field on the optical axis of the ion beam or a fluctuation of the magnetic field.
To address these needs, Patent Document 4 discloses a technique to configure an optical system. The technique involves producing a corrective magnetic field on the optical axis of the ion beam. The corrective magnetic field causes a deflection by which the deflection of the ion beam due to an external magnetic field on the optical axis or a leakage magnetic field from the SEM objective lens can be cancelled, so that the beam spot position on the sample can coincide with the spot position in the case of absence of the magnetic field. In this configuration, even when the ion beam comprises a plurality of isotopes, with their different optical paths introduced by separation, the ion beams of all of the isotopes are focused back at one point on the sample in terms of the beam spot position.
Patent Document 1: JP Patent Publication (Kokai) No. 11-329318 A
Patent Document 2: JP Patent Publication (Kokai) No. 11-329320 A
Patent Document 3: JP Patent Publication Kokai) No. 7-296756 A
Patent Document 4: JP Patent Publication (Kokai) No. 2006-40809 A